Selina: The program. The 'Clean Slate'. Where is it?
Daggett: The 'Clean Slate'? Where you type in someone's name, date of birth and a few minutes their gone from every database on earth? Sound a little too good to be true.
Selina: You're lying! Rykin Data took it to prototype stage.
Daggett: That's why I bought 'em. But they had nothing. It was a gangland myth.

What does gangland myth mean in general?
It was a myth common in gangland (among gang members).